This chapter discusses the fundamental thermodynamic concepts such as laminar flow and turbulent flow. Flow tends to be turbulent in upper airway obstruction, so gas density is influential. The functional anatomy of the upper airway can be reduced to a consideration of a collapsible segment (the pharynx) between two rigid segments (the nasopharynx and the trachea). This system behaves as a Starling resistor and airflow can become limited or completely abolished during spontaneous (negative intrathoracic pressure) breathing. Maintenance of pharyngeal airway patency is a complex neuromuscular phenomenon. In airway obstruction at the pharyngeal level, inspiratory flow may not be increased by increased inspiratory effort, but can be increased by positive pressure applied above the obstruction. The chapter describes the physics and function of a device which permitted ventilation of patients during bronchoscopy. There are a number of misconceptions regarding the operating principles of the Sanders injector.